1. Field of the Invention
This invention relates to an optical communication system, and more particularly, to a device (multiplexer/demultiplexer) for compounding/dividing light depending on an optical wavelength.
In an optical wavelength multiplex communication, a plurality of light beams having different wavelengths (respective wavelength channels) which communicate a plurality of different information are compounded in a single mode fiber, and this information is communicated therethrough.
Therefore, it is necessary to divide the composite lights into respective light beams depending on the wavelength channels thereof at the receiving station, to obtain the information loaded on the respective wavelength light beams.
In such a system, an optical wavelength compounding/dividing device is used for compounding the light beams having a plurality of wavelengths in a single mode fiber and for dividing the composite lights depending on the respective wavelengths.
2. Description of the Related Art
FIG. 8 is a side cross-sectional view of a conventional optical system for compounding/dividing light depending on an optical wavelength. Here, it is presumed that this system is used as a demultiplexer for dividing light beams.
This optical system is provided with an input optical fiber 21, a spectroscopic grating 22, an output fiber array 23, and a collimator lens 24. The light beam input from the optical fiber 21 is changed to parallel light beams by the lens 24 and reflected by the grating 22. The light beams diffracted on different wavelengths by the grating 22 are focused on the respective optical fibers of the output fiber array 23 arranged according to the respective wavelength channels and transmitted therethrough.
However, in a conventional optical system as mentioned above, there is an serious drawback in that the open regions of the output optical fiber core available for the respective wavelengths are relatively narrow, compared to the pitch between adjacent fibers of the output fiber array. Therefore, only light beams in the narrow wavelength region within a same wavelength band can be picked up by the output fibers.
FIGS. 9, 10, and 11 show another conventional optical system known in the prior art and disclosed in Japanese Unexamined Patent Publication No. 60-107004 and "Electronics Letters", Vol. 21, No. 10, page 423, 1985, by I. Nishi et al. This optical system is provided with an input optical fiber 31, a self-focusing lens 32, a spectroscopic grating 33, upper and lower rectangular prisms 34a and 34b, first and second output fibers 35a and 35b, and a glass block 36. FIG. 10 shows the relationship between the shift of the incident light position in accordance with the wavelength and the light beams emitted from the upper and lower rectangular prisms 34a and 34b. FIG. 11 schematically shows the change of light beam position due to the wavelength thereof.
The light input from the optical fiber 31 is changed to parallel light (FIG. 11a) by the self-focusing lens 32 and input via the glass block 36 to the grating 33 inclined by a predetermined angle. The light is diffracted by the grating 33 depending on the wavelength thereof (FIG. 11b). Diffracted light in a certain wavelength region, such as .lambda..sub.1 to .lambda..sub.2, is focused on the upper rectangular prism 34a and emitted in the opposite direction through the self-focusing lens 32. Next, the light is introduced to the grating 33 to be diffracted again and is then focused on the first output fiber 35a through the self-focusing lens 32. Similarly, light in another wavelength region, such as .lambda..sub.3 to .lambda..sub.4, is focused on the lower rectangular prism 34b and focused on the second output fiber 35b. Therefore, the positions of light beams emitted from upper and lower rectangular prisms 34a and 34b are as seen in FIG. 11c. Also, the positions of light beams focused on the first and second output fibers 35a and 35b are as seen in FIG. 11d.
This optical system makes it possible to broaden the bandwidth in respective wavelength channels, when compared with an optical device such as shown in FIG. 8. However, it would be difficult to increase the number of wavelength channels to realize a multi-channel optical device.